Hyperthermia combined with radiation has been effective in the treatment of some tumors, despite the limited capabilities of the available heating technologies. Clinical temperature distributions have often been nonuniform, and temperatures below the therapeutic range have frequently been measured within the treated volume. A significant cause of this variability is the strong dependence of the steady-state temperature on blood perfusion rate, which can vary considerably among tumors, and among locations within a single tumor. One possible solution to this problem is to use very short heating times, since the initial temperature elevation does not depend strongly upon blood perfusion rate at the beginning of energy input. By delivering the required thermal exposure in a short time period (an period which would require relatively high temperatures) it may be possible to cover the whole tumor with a therapeutic thermal exposure. (The proposed technique is not equivalent to cauterization since the thermal exposure will be designed to be equivalent to that given with standard hyperthermia). An additional potential benefit is that, this approach may eventually led to a therapy where none or only a few, invasive temperature probes, are required. Also, it is important to note that this kind of treatment can be delivered using existing technology. In this study we propose to utilize focused ultrasound to induce the high temperatures required. The applied ultrasonic field will be optimized and the effect of variations in the thermal and acoustical properties to tissues on the temperature elevation will be investigated using computer simulations, and tests both in vitro and in vivo perfused organs. Also the significances of nonlinear propagation, transient cavitation, tissue interfaces and large blood vessels will be studied in vivo. Finally, if these results indicate, the feasibility and potential as well as the toxicity and maximum tolerated thermal exposure of the high temperature hyperthermia will be evaluated in a clinical phase I/II trial.